The process of adding harmonic distortion, particularly even harmonic distortion, to improve the clarity of audio signals is known in the field of audio signal processing. U.S. Pat. Nos. 3,828,133; 4,150,253; 5,133,014; and 5,359,655 disclose processes in which the audio signal is divided into two signal paths. One signal path is utilized to generate the harmonic distortion which is then combined with the other signal path to produce and enhance the audio signal. The combined signal is perceived to be of higher quality when even harmonic distortion is used. This is particularly true when the audio signal is a voice signal.
Processes for producing even harmonic distortion have been employed in wide bandwidth (20 Hz to 20 KHz) signal processing, i.e. high fidelity audio signals. For example, U.S. Pat. No. 5,424,488 discloses a harmonics generator which may be used to improve the sound quality of electronic audio equipment.
Harmonic distortion has also been used to clarify certain bandwidth limited signals, such as telephone audio signals. A typical bandwidth for a telephone signal is about 4 KHz, ranging from about 150 Hz to 3900 Hz. Prior art systems for use in telephony applications improve clarity of the audio signal by adding even harmonics of particular frequencies at the receiver so as to "fill-out" the spectrum which was limited by the communication channel. Thus, the added harmonic frequencies are typically outside of the 4 KHz bandwidth of the telephone transmission line. As such, this method requires the audio processing to be performed at the receiver; otherwise, any added distortion would be filtered out by the transmission channel.
It is desirable to provide a system and method which may be used to clarify limited bandwidth audio signals at any point in the transmission path. In telephony applications, this allows selective utilization and control of the clarification process by individual customers.
It is further desirable to provide a system and method which reduces the number of electronic components necessary to impart even harmonic distortion to an audio signal by providing a single signal path. This allows miniaturization of the system for appropriate applications, such as use in hearing aids and the like.